How do I create a magnified glass effect in Cycles?
The Glass BDSF material in the Cycles renderer does this quite nicely. The controlling variable is the IOR (index of Refraction). Set it to 1.0 and the lens will act like it is made of air (no effect, in essence). Keep it at the default 1.45 and apply the material to a lens-shaped object, and you should see the effect you are looking for. Play around with IOR until you get what you need.